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Introduction to Pinch Technology

Rokni, Masoud

Publication date:2016

Document VersionPeer reviewed version

Link back to DTU Orbit

Citation (APA):Rokni, M. (2016). Introduction to Pinch Technology. Kgs. Lyngby: Technical University of Denmark (DTU).

Introduction to Pinch Technology

Division of Energy Section Masoud Rokni

Technical University of Denmark MR@mek.dtu.dk

DTU Building 402, 1st Floor

DK-2800 KGS, Lyngby

July 2006

Revised April 2016

1

2

Introduction to Pinch Technology

1. Introduction

The demand for effective usage of energy processes is increasing and nowadays every

process engineer is facing the challenge to seek answers to the questions related to their process

energy patterns. Below the questions that frequently may be asked are listed:

Is the existing process as energy efficient as it should be?

How can new projects be evaluated with respect to their energy requirements?

What changes can be made to increase the energy efficiency without incurring any cost?

What investments can be made to improve energy efficiency?

What is the most appropriate utility mix for the process?

How to put energy efficiency and other targets like reducing emissions, increasing plant

capacities, improve product qualities etc, into a one coherent strategic plan for the overall

site?

All these questions and more can be answered with a full understanding of Pinch Technology

and an awareness of the available tools for applying it in a practical way. The aim here is to

provide the basic knowledge of pinch technology concept and how it can be applied across a

wide range of process industries. The pinch technology was proposed firstly for optimization of

heat exchangers and therefore it is introduced / described below for such devises. Heat exchange

equipments encounter in many industries for at least two reasons; a) it is often necessary as part

of the process to change the thermal condition and b) it is the ambition to minimize the energy

consumption of the given process. With other words the idea is to maximize the energy recovery

within the process or to minimize the use of external energy sources. Figs 1 and 2 illustrate this

meaning.

a) b)

Figure 1. a) Process with only external energy sources and b) Process with internal and external

energy sources.

HEX HEX

HEX

HEX

HEX HEX

HEX

HEX

HEX

HEX

3

In Fig. 1a heat is added or removed only with external sources (heaters and coolers) while the

same process can be improved by using internal heat exchanges under the condition of an overlap

in the temperature intervals, see Fig 1b. The need of external energy sources can be reduced by

using internal heat exchanges between cold and hot streams and a more energy efficient process

can thus be achieved. However, it does not mean that the total cost is reduced since the use of

internal heat exchange results in increased capital cost through bigger or more heat exchanger

units. Fig. 2 shows how heat can be recovered in s simple process scheme by using internal heat

exchange.

a)

b)

Figure 2. A simple process a) with external energy sources only and b) with internal and external

energy sources.

The application of such optimization method is referred as “Pinch Technology” that

guarantee minimum energy levels in design of heat exchanger network in a process.

2. Problem Statement

A typical industrial process may consist of several numbers of hot and cold process streams

which may demand cooling and heating respectively. Heat exchangers can be used to recover

some of the heat demand while external heaters and coolers can be used to achieve the

temperature demand of the process streams. Suppose an industrial plant with hot and cold process

Product

1 Reactor

200C 2

Feed

Steam

Cold Water

80C 90C

80C

Product

1 Reactor

200C

3

2

Feed

Steam

Cold Water

90C

4

streams as shown in table 1. Heat capacity rate is defined as mass flow rate times heat capacity

pCm

.

Table 1. Process streams in an industrial plant.

Process stream

Nr / Type

Inlet Temp.

[C]

Outlet Temp.

[C]

Heat capacity rate

[kW/K]

Q [kW]

1. cold 90 420 10 3300

2. cold 170 350 32 5760

3. cold 200 390 29 5510

4. hot 440 140 27 8100

5. hot 510 300 24 5040

The task is to find the optimal network of heat exchangers, external coolers and external

heaters with respect to the capital and annual operating cost. The maximum heat that can be

transferred in a heat exchanger is limited by the minimum allowed temperature difference

between hot and cold streams, called as Tmin. The temperature level at which Tmin is observed

is called as “pinch point” and the analysis to find this temperature with respect to the laws of

thermodynamic is called as “Pinch Analysis” or “Pinch Technology”. Capital costs depend

mainly on the number of heaters, coolers, heat exchangers and their sizes (area) while operating

cost is mainly dominated by the need for external energy supplied such as heating and cooling.

Thus the main objective of pinch analysis is to achieve financial savings by better process heat

integration (maximizing the process heat recovery and reducing the external utility loads).

In table 1, the heating effect and cooling effect under steady state condition (constant heat

capacities and temperatures) are

Qheat = 10 x (420 – 90) + 32 x (350 – 170) + 29 x (390 – 200) = 14570 kW

Qcool = 27 x (510 – 300) + 2.0 x (440 – 140) = 13140 kW

which must be supplied by external heater and cooler such as steam water and cold water. The

idea is now to find the lowest possible Qheat for this particular problem.

3. The Pinch Method

The pinch method consists of two steps. In the first step the maximum energy recovery in the

system should be calculated which in turn reduces the external energy needed to a minimum

level.

Consider the process streams and it temperatures in table 1. Figure 3 shows the process

streams and the temperature intervals for the plant. The inlet and outlet temperatures can now be

divided into temperature intervals where the first temperature interval is between the largest

temperature and the second largest temperature. The next interval is between the second largest

temperature and the third largest temperature, etc. Such temperature intervals results are shown in

table 2.

For a given interval, one may calculated the amount of heat to be supplied to the plant (called

as Qheat) and how much how much heat must be taken from the plant (called as Qcool). The

relationship between these external cooling and heating can be written as

heatcool QQQ

. (1)

5

Figure 3. Temperatures of the process streams.

Table 2. Temperature interval and external heating and cooling effects.

Interval

Number

Temperature

interval [C]

Stream

Numbers coolQ

[kW]

heatQ

[kW]

Q

[kW]

1 510 – 440 5 1680 0 1680

2 440 – 420 4 + 5 1020 0 1020

3 420 – 390 1 + 4 + 5 1530 300 1230

4 390 – 350 1 + 3 + 4 + 5 2040 1560 480

5 350 – 300 1 + 2 + 3 + 4 +5 2550 3550 - 1000

6 300 – 200 1 + 2 + 3 + 4 2700 7100 - 4400

7 200 – 170 1 + 2 + 4 810 1260 - 450

8 170 – 140 1 + 4 810 300 510

9 140 – 90 1 0 500 - 500

For example in interval 4 where the temperature interval is between 390C to 350C, the cooling

and heating demands are:

Qheat = 10 x (390 – 350) + 29 x (390 – 350) = 1560 kW

Qcool = 27 x (390 – 350) + 24 x (390 – 350) = 2040 kW

Q = Qcool – Qheat = 2040 – 1560 = 480 kW

It means that a cooling effect of 480 kW must be supplied to the interval 4 so that the process

stream can deliver the lowest needed temperature which is 350C.

T [C]

0 100 200 300 400 500 600

1

2

3

4

5 24

27

29

32

10

pCm

6

Another important conclusion from table 2 is that in interval 6 the need for external heating is

– 4400 kW which is the largest heating demand and must be supplied by external heating to the

process. In pinch method this external heating must be first supplied to the system at the

respective temperature interval and then all other temperature intervals must be updated

according to this heating demand.

There are two major methods to calculate the pinch point, table method and composite curve

method. The table method will be discussed next. However, in between the importance of Tmin

is going to be discussed first.

3.1 The meaning of Tmin

The interval and calculated heat transfer in table 2 requires an ideal heat transfer within a heat

exchanger that cool the hot stream down to the minimum temperature difference of Tmin = 0. It

means that the heat exchanger area (size) is infinite,

Tmin 0 heat exchanger size and price .

This of course is not possible in practical applications and Tmin ≠ 0 is always valid. In order to

decrease the size of heat exchanger to an acceptable level with reasonable price it is assumed that

there always exists a temperature difference, preferably Tmin = 10C. This 10C can be treated

in three different ways. It can be added to the cold streams, which means that the cold streams are

warmed with 10C. Or, it can be divided between cold and streams which mean that the cold

stream will be warmer by 5C and the hot streams will be cooler by 5C. Finally, it can be

decreased from hot streams which in turn mean that the hot stream will be cooled by 10C.

Suppose that we choose the last option, although the hot streams cannot be fully warmed and

they will be cooler by 10C. A new temperature interval can thus be calculated which is specified

in table 3.

Table 3. Temperature interval for Tmin = 10C.

Interval

Number

Temperature

interval [C]

Stream

Numbers coolQ

[kW]

heatQ

[kW]

Q

[kW]

1 500 – 430 5 1680 0 1680

2 430 – 420 4 + 5 510 0 510

3 420 – 390 1 + 4 + 5 1530 300 1230

4 390 – 350 1 + 3 + 4 + 5 2040 1560 480

5 350 – 290 1 + 2 + 3 + 4 +5 3060 4260 -1200

6 290 – 200 1 + 2 + 3 + 4 2430 6390 -3960

7 200 – 170 1 + 2 + 4 810 1260 -450

8 170 – 130 1 + 4 1080 400 680

9 130 – 90 1 0 400 -400

3.2 The Problem Table Algorithm (PTA)

In this method the temperature intervals can be described as thermal blocks connected to each

other in series which is illustrated in Fig. 4. Every temperature interval can be regarded as a

7

network (or sub-network) that could be optimized with respect to the maximum energy recovery

within the process. Such coupling is also called as cascade coupling.

Figure 4. Illustration of serial network connection.

The excess energy, which is usually called as deficit, for an arbitrary sub-network i can be

obtained from

hot

p

cold

piii CmCmTTD 1 (2)

where Ti and Ti+1 corresponds to upper and lower temperatures in the arbitrary temperature

interval i. The important following condition is thus valid

Di < 0 need for cooling

Di > 0 need for heating

The energy balance for the arbitrary block i can be calculated by

iiii DQQ

1,11, (3)

where Qi-1,i and Qi,i+1 are the supplied heat and removed heat respectively for each block. Table 4

shows the calculated supplied and removed heat for each temperature interval (block) as given in

table 1 and figure 3. Initially the supplied heat Q0,1 for block 1 is set to Q0,1 = 0.

The supplied heat Qi-1,i and the removal heat Qi,i+1 for any interval i are usually called as

sequential balance, which is illustrated in table 4. From table 4 it can be seen that Q6,7 , Q7,8 , Q8,9

and Q9,10 have negative values. These values are unreasonable because it would mean that heat is

moving from a lower temperature to a higher temperature. In order to create a reasonable

network, first the highest negative value must be found and then all the sequential balance values

must be added with this highest negative value (mathematically the lowest value). In the table, we

shall see that the highest negative value is – 1710 kW and this value must now be added to the

sequential balance values of Qi-1,i and Qi,i+1. The updated results are shown in the same table

(table 4) and are called as max table which refers to the last two columns at table 4. Table 4 is

usually called as cascade table.

From heater To cooler

Qn,n+1 Qn-1,n Qi,i+1 Qi-1,i Q2,3 Q1,2 Q0,1

T

1 2 i n

D1 D2 Di Dn

8

From the last two columns of the table (Max table) three important values can be read. First,

the minimum external heat supply to enable the network working is read as 1710 kW. Second,

the minimum external cooling is 280 kW. Third, there exists a point where the heat flow between

the points is zero. This point is called the “pinch point” of the system.

Table 4. Sequential balance problem for the case in table 1 with Tmin = 10C.

Sequential Balance Max Table

Interval Temp.

limits

Di iiQ ,1

1,

iiQ iiQ ,1

1,

iiQ

1 500 – 430 -1680 0 1680 1710 3390

2 430 – 420 -510 1680 2190 3390 3900

3 420 – 390 -1230 2190 3420 3900 5130

4 390 – 350 -480 3420 3900 5130 5610

5 350 – 290 1200 3900 2700 5610 4410

6 290 – 200 3960 2700 -1260 4410 450

7 200 – 170 450 -1260 -1710 450 0

8 170 – 130 -680 -1710 -1030 0 680

9 130 – 90 400 -1030 -1430 680 280

3.3 Importance of Pinch Point

Identification of the pinch point is very important when the process system consist of several

heat exchangers, coolers and heaters, in order to recover energy within the system with maximum

results and decrease the need for the external heating and cooling energy. Thus energy target can

be realized by designing an appreciate heat recovery network. The following rules immediately

are valid

- External coolers cannot be used above the pinch point (otherwise they should be heated

again)

- External heaters cannot be used below the pinch point (otherwise they should be cooled

again)

- Heat exchanger cannot be used across the pinch point because otherwise all heat flows

must be increased with this heat transferred.

Violation of any above rules results in higher energy requirements than the minimum

requirements theoretically possible. These rules are often called as pinch rules.

The pinch point divides the process into two separate systems which means that the areas

below and above the pinch point should be designed separately, when designing the process

network. Violation of any rules mentioned above results in higher energy requirements than the

minimum energy requirements that theoretically would be possible. Furthermore the following

conditions should be fulfilled at design closest to the pinch point

- cold

p

hot

p CmCm

immediate above the pinch point (4a)

- cold

p

hot

p CmCm

immediate below the pinch point (4b)

9

Due to several reasons such as safety reasons, corrosive media, pressure level, etc. some heat

exchangers cannot be used for some streams. Such limitations must be overcome with special

treatment, which are out of the scope of here and will not be further discussed.

3.4 Importance of Tmin

The driving force for heat transfer in a heat exchange is the average temperature difference

Tmin through Q = UATmin, where A is the heat exchanger area and U is the heat transfer

coefficient. For a given heat transfer coefficient the need for heat transfer area decreases with

increasing Tmin. For example, a heat exchanger with Tmin = 10C is about 90% smaller than a

heat exchanger with Tmin = 1C. Further, for Tmin = 20C the cost for external energy will

increases while the capital cost decreases. In addition, a balance between capital cost (heat

transfer area) and energy cost (external Qheat and Qcool) decides the optimum Tmin, as shown in

Fig. 5.

The capital cost for heat exchangers may have the form of a Area b + c while the energy cost

may be approximated as linear function of d (Qheat) + e (Qcool) which is a function for cost applied

to annual cost.

Figure 5. Different cost as function of Tmin.

Note that an increase of Tmin does not necessary give the same relative increase in

temperature difference between the streams everywhere. The biggest relative increase is obtained

close to the pinch point while the relative increase may be marginal away from this optimum

point.

3.5 Composite Curves

The problem with energy recovery within the system can also be illustrated graphically by

plotting the temperature – enthalpy curves. In order to do that the cold and hot streams shall be

Optimum Tmin

Energy cost

Tmin [C]

Capital cost

Total cost

Cost

10

treated separately and the temperature intervals may be decided similar to the problem table

algorithm method. Consider the streams for the process defined in table 1. The temperature

interval for the hot stream can be illustrated as shown in Fig. 6. Suppose that the temperature

interval is beginning from the lowest temperature to the highest temperature. The temperature

intervals with respective streams, heat capacity rates and enthalpy are also shown in table 5.

Three temperature intervals are found.

Figure 6. Temperature intervals for the hot streams from table 1.

Table 5. Temperature intervals and enthalpy flow rates for hot streams only.

Interval

Number

Temperature

interval [C]

Stream

Numbers

PCm

[kW/K]

PCmTH

[kW]

H

[kW]

1 140 – 300 4 27 4320 4320

2 300 – 440 4 + 5 51 7140 11460

3 440 – 510 5 24 1680 13140

Figure 7. Composite curve for hot streams.

T [C]

0 100 200 300 400 500 600

4

5 24

27

pCm

140C 300C 440C 510C

4320 7140 1680

Enthalpy [kW]

T [C]

0 3000 6000 9000 12000 0

50

100

150

200

250

300

350

400

450

500

550

15000

11

In table 5, the enthalpy flow for any temperature interval can be calculated as the product of the

temperature difference and the sum of the heat capacity rates. Now the composite curve for hot

streams indicating the enthalpy – temperature diagram can be drawn for each temperature. Since

enthalpy is a relative value then the enthalpy for the first point (T = 140C) can be assumed to be

zero. Of course any other values can also be assumed. The results are shown in Fig. 7.

The cold streams can also be treated in the same way, see below. The illustration of the

temperature interval for the cold streams can be found in Fig. 8. The temperature interval for the

cold streams including the heat capacity rates and enthalpy are show in table 6. Five intervals are

found and for each temperature interval the enthalpy is the sum of all enthalpy for the previous

enthalpies.

Figure 8. Temperature intervals for the cold streams from table 1.

Table 6. Temperature intervals and enthalpy flow rates for cold streams only.

Interval

Number

Temperature

interval [C]

Stream

Numbers

PCm

[kW/K]

PCmTH

[kW]

H

[kW]

1 90 – 170 1 10 800 280 – 1080

2 170 – 200 1 + 2 42 1260 1080 – 2340

3 200 – 350 1 + 2 + 3 71 10650 2340 – 12990

4 350 – 390 1 + 3 39 1560 12990 – 14550

5 390 – 420 1 10 300 14550 – 14850

The temperature – enthalpy diagram can now be included in composite curve of the hot streams.

The results are shown in Fig. 9. Note that from table method the external cooling was determined

to be

280

coolQ kW

T [C]

0 100 200 300 400 500 600

1

2

3 29

32

10

pCm

90 170

200

350

390

420

12

which is also the starting value (enthalpy) for the cold stream at 90C.

Figure 9. Composite curves for hot and cold streams.

The pinch point in the composite curve is the point where the smallest temperature difference

(Tmin) between the hot composite and cold composite curves as indicated in Fig. 9. The external

cooling and external heating are the difference between the curves under and above pinch point

respectively. The overlapping area between the hot composite and cold composite curves is the

heat duty which is transferred in the heat recovery heat exchangers. The placement of the cold

composite curve is determined by the magnitude of the Tmin. Increasing the Tmin means that the

cold composite curve is shifted sideways in the enthalpy direction, see Fig. 10.

Figure 10. Composite curves when the Tmin increasing.

Cold composite

Hot composite

Enthalpy [kW]

T [C]

0 3000 6000 9000 12000 15000 0

100

200

300

400

500

QHeat Exchangers

Qheat

Cold composite

Hot composite

Pinch

point

Enthalpy [kW]

T [C]

0 3000 6000 9000 12000 15000 0

50

100

150

200

250

300

350

400

450

500

550

Qcool

13

3.6 Grand Composite Curve (GCC)

The cascade coupling of the heat flows shown in Fig. 4 can graphically be illustrated in a

temperature – heat flow (or enthalpy) diagram where each heat flow can be drawn against its

respective temperature interval as shown in Fig 10. This Grand Composite curve is drawn for

Tmin = 10C as was assumed in table 3 previously. The results for table 4 are graphically shown

in this figure. The GCC gives more detailed information about the process streams, external

heater and cooler utilities, etc. compared to the composite curves.

Figure 11. “Grand Composite” curve.

280

680

0

450

4410

5610

5130

3900

3390

Qheat

Qcool

Pinch

Qheat

Qcool

Heat flow [kW]

0 1200 2400 3600 4800 6000 0

50

100

150

200

250

300

350

400

450

500

550 T [C]

- 1680

- 510

- 1230

- 480

1200

3960

450

- 680

400

1

2

3

4

5

6

7

8

9

1710

14

4. Design of Heat Exchanger Network (HEN)

The design of new heat exchanger networks can be best executed by using the pinch method.

Using the pinch method incorporates two important features: a) it recognizes that the most

constrained part of the problem is the pinch region and b) designers are allowed to choose

between the match options. The designer examines which hot streams can be matched to the cold

stream by heat recovery. Every match brings one stream to its target temperature and the pinch

separates the heat exchanger systems into two thermally independent regions, heat exchange

networks above and below pinch temperature. When the heat recovery is maximized the

remaining thermal needs are supplied by external heat utility.

There is not any clear method in designing the HEN and therefore the designer shall try

different networks and finally find the optimal one. However, it is important to remember some

rules when designing the HEN

The network above and below the pinch shall be designed separately (independently)

Start with placing a heat exchanger close to the pinch point and continue outwards

Allow the heat exchangers transfer heat as much as possible

The need of heat load for hot streams shall be satisfied above the pinch point

The criteria (4a) and (4b) mentioned previously are fulfilled

Sometimes it is necessary to split one stream in order to mach some loads. The following

diagram may help in splitting the streams. Suppose Nhot and Ncold are the number of hot and cold

streams respectively.

a) b)

Figure 12. Flow diagram for splitting of streams a) above the pinch b) below the pinch.

In order to design the HEN for the example above it is useful to create tables in which the streams

specifications above and below the pinch temperature are shown, see tables 7 and 8. Note that the

pinch temperature was 170C, therefore, above the pinch the cold streams shall be heated from

170C while the hot streams must be cooled to 180C (if Tmin = 10C). In these tables such

aspects are also taken into account. As shown in the table there exist only one cold stream and

one hot stream below the pinch point (streams number 1 and 4).

Nhot Ncold

Yes No

Split one hot

stream

cold

p

hot

p CmCm

for every pinch match ?

Split one stream

(usually cold)

No Yes

Below

Pinch

Nhot Ncold

Yes No

Split one cold

stream

cold

p

hot

p CmCm

for every pinch match ?

Split one stream

(usually hot)

No Yes

Above

Pinch

15

Table 7. Process streams above the pinch with Tmin = 10C .

Process stream

Nr / Type

Inlet Temp.

[C]

Outlet Temp.

[C]

Heat capacity rate

[kW/K]

Q [kW]

1. cold 170 420 10 2500

2. cold 170 350 32 5760

3. cold 200 390 29 5510

4. hot 440 180 27 7020

5. hot 510 300 24 5040

Table 8. Process streams below the pinch with Tmin = 10C.

Process stream

Nr / Type

Inlet Temp.

[C]

Outlet Temp.

[C]

Heat capacity rate

[kW/K]

Q [kW]

1. cold 90 170 10 800

2. cold – – – –

3. cold – – – –

4. hot 180 140 27 1080

5. hot – – – –

Figure 13. Grid diagram above the pinch point.

249

180C

393.3

T [C]

170 240 320 400 480 560

5

4

3

2

1

27

29

10

32

PCm

stream

5760 470

5040

790

1710

24

16

Immediate above the pinch point, streams 4 and 2 fulfill the condition (4a), and table 7 shows

that the heat available from hot stream 4 can satisfy the needed heat for cold stream 2. The rest of

1260 kW (7020 – 5760 = 1260 kW) can be used for heat supply to other cold streams. An energy

balance provides the breaking temperature for stream 4,

5670 x 103 = 27 x 10

3 (T – 180) T = 393.33C

These temperatures are far away from the pinch point and the condition (4a) is not in use any

more. Therefore, the rest heat of 1260kW can be used for any other cold streams. The hot stream

5 can provide 5040 kW needed heat for cold stream 3. Again, temperature 300C for stream 5 is

far away from pinch ant the condition (3a) is not valid any longer. Now the cold stream 3 needed

470 kW (5510 – 5040 = 470) which can be taken from rest of stream 4. After this, stream 4 has

790 kW left (1260 – 470 = 790) which can be given to the cold stream 1. The breaking

temperature for stream 1 can be calculated by

790 x 103 = 10 x 10

3 (T – 170) T = 249C

The cold stream 1 needed now 1710 kW (2500 – 790 = 1710), which can be provided by external

heating. No split for any streams was needed since the conditions (4a) and (4b) were fulfilled

completely. The external heating is the same as was calculated previously. The results are

graphically illustrated in Fig. 13 which is usually called as Grid Diagram.

A similar treatment can also be done for stream below the pinch point, which is illustrated in

Fig. 14. The cold stream 4 needs 800 kW which can be provided by the hot stream 1 due to the

fact that the condition (4b) is fulfilled. The breaking temperature can be calculated through

energy balance equation as

800 x 103 = 27 x 10

3 (180 – T) T = 150.37C

The hot stream 4 needs now 280 kW (1080 – 800) which can be supplied by the external cooling

utility.

Figure 14. Grid diagram below the pinch point.

Now if grid diagrams in Figs. 13 and 14 are connected then the whole network can be graphically

shown in Fig. 15. Note that it is always useful to show the breaking points for the respective

stream in the grid diagrams in order to more exactly specify where the considered heat

exchangers are place. Another advantage is that by showing these breaking temperatures the

designer can easily realize how far these temperatures in comparison to the pinch point are place

and thus to understand whether the conditions (4a) and (4b) are relevant. Note that these

conditions are only valid immediately above or below the pinch point, not every where else.

150.37

90 120 150 180

PCm

stream

1

4 27

10

T [C]

800

280

17

Figure 15. Resulting grid diagram for the network.

4.1 Minimum Number of Heat Exchangers

At this point it might be interesting to mention the minimum number of heat exchangers units

that can be used in HEN designing. Below the pinch point the heat exchangers load is chosen so

that the cold streams are satisfied while above the pinch point the heat exchangers load is chosen

so that the hot streams are satisfied. As mentioned previously, to fulfill these conditions it is

sometime necessary to split some streams. Further, no heat transfer is allowed across the pinch

point in designing the Minimum Energy Requirement (often called as MER). Therefore, a

realistic target for the minimum number of units (UtargetMER) would be the sum of targets

evaluated at both above and below the pinch point separately as (Euler’s theory),

BelowPinchutilitycoldhotAbovePinchutilitycoldhotetMERt NNNNNNU 11arg (5)

where Nhot , Ncold and Nutility are number of hot streams, number of cold streams and number of

utilities (heaters and coolers) respectively. For the example above, below the pinch there are 2

streams and 1 heater, thus the minimum number of heat exchangers would be 2 +1 – 1 = 2 which

is also used in this HEN. Above the pinch, there are 5 streams (3 cold plus 2 hot) and 1 cooler

which results in 5 + 1 – 1 = 5 which is the minimum HEN and also is used in this solution HEN.

Note that the external cooling (1710 kW) and heating (280 kW) shall also be provided by heat

exchangers.

Moreover, the cost of heat exchangers should be in balance compared to other solution

methods such the need for more external heating and cooling. The target for minimum heat

transfer area and minimum number of heat exchanger units can be combined with the heat

373.79

249

150.37 393.3

T [C]

100 200 300 400 500

1

2

3

4

5 24

27

29

32

10

stream PCm

180

170

280

1710

5760

5040

790 800

470

18

exchanger cost method to determine the target for the HEN capital cost. Note that the minimum

of units is of more interest in respect for cost of pipelines, fundaments, maintenance, etc.

5. Pinch Technology and Additional Points

As pointed out before the pinch technology offers a novel approach on designing a process

with maximal energy recovery network and thus minimizing the capital and maintenance cost.

Here some additional points will be pointed out which the designer shall be think.

5.1 Pinch Technology and Traditional Approach

In traditional designing approach, the core of process is designed with fixed flow rates and

temperatures so that the heat and mass balance of the process is satisfied. The design of heat

recovery systems are provided afterwards and the remaining duties are competed by using the

external utilities. In pinch technology approach integration of heat recovery systems is considered

together with process designing. A simple map in Fig. 16 shows clearly the differences.

Figure 16. Traditional design approach versus pinch design approach.

5.2 Guidelines on Deciding the Magnitude of Tmin

In case information lack, the table below shows some guidelines on preliminary assumption

of Tmin for some different industrial processes.

Table 9. Some typical values for Tmin in industrial sectors.

Industrial Sector Typical values for Tmin

Low Temperature Process 3 – 5 C

Chemical 10 – 20 C

Petrochemical 10 – 20 C

Oil Refinery 20 – 40 C

Design of

Core

Process

Heat Recovery

Heat

Exchangers

Utility Systems

Conventional Approach

Pinch Technology Approach

Heat Recovery

Heat

Exchangers

Design of

Core

Process Target

s Target

s

Target

s

Utility Systems

19

Note that the Tmin should be highly dependent on the flow type, heat exchanger type, sector to

be used, etc. For example, in oil refinery fouling is problem in the heat exchangers and therefore

Tmin would be large. Or power requirement for refrigeration process is very expensive and

therefore low value for Tmin is desired.

5.3 Steps of Pinch Analysis

A well defined stepwise procedure shall be followed in any pinch analysis problem, whether

it is a new project or it is retrofit situation (existing project to be updated). Note that it might be

necessary to iterate between some steps before continuing. Re-simulation and data modification

may also be useful in many situations.

- Identification of the cold streams, hot streams and utilities

- Thermal data extraction for the streams and utilities

- Tmin value selection

- Composite curves and grand composite curve construction

- Minimum energy cost target estimation

- Capital cost target estimation

- Optimum Tmin value estimation

- Practical targets for HEN design estimation

- HEN design

5.4 Benefits and Application Areas of Pinch Technology

Maybe one of the main advantages of the pinch technology over the traditional methods is the

ability of the pinch technology to set capital cost and energy recovery targets for a sub-process or

entire process ahead of design. Also, before designing any process the scope of energy savings

and investment requirements are known in advanced. In addition, the following process

improvements are among many that the pinch technology helps the process engineers to achieve.

- Updating/modifying the existing process flow diagram. It shows the areas that process

changes reduce the overall energy target.

- Process simulation studies. Old energy studies can be replaced by pinch technology

simulations so that information can be easily updated. It helps to avoid unnecessary

capital cost before the projects are implemented.

- Practical targets settings. Theoretical targets can be modified by taking into account

practical restrictions (safety, difficult handling fluids, temperature limit, pressure limit,

etc.).

- Determination of opportunities for Combined Heat and Power (CHP) process. Power cost

reduces significantly with a well designed CHP system. Pinch technology shows the best

possible CHP system that matches the thermodynamic targets on the site.

- Deciding what can be done with the low-grade waste heat. Pinch technology shows

which waste heat streams can be recovered effectively.

6. Pinch Method with Active Components of Heat Engines and Heat Pumps

Heat engines and heat pumps are the key components for the process utility systems. The idea

would therefore be the principles of placing these components into the process, or appropriate

integration of such devices into the process. Figure 17 shows the principle working mechanism

20

of heat engines and heat pumps. A heat engine receives heat from a high temperature source and

produces work while emitting heat to a lower temperature source. A heat pump (refrigeration

mechanism) takes heat from a lower temperature reservoir by supplied work and rejects heat to a

high temperature source. The following relation is valid

21

QQW (6)

a) b)

Figure 17. Principles mechanism of a) heat engines and b) heat pumps (refrigeration).

6.1 Heat Engine (HE)

A heat engine in a process has two main objectives, supplying process heat demand and

generating power. An appropriate integration of heat engines into a process is important on

providing the most energy efficient combination of these objectives. By integration it means that

a heat exchange link is provided between the heat engine and the process. The integration of heat

engines into a process can be done on three different ways, above the pinch, below the pinch and

across the pinch. These integration possibilities are shown in Fig. 18.

a) Above the pinch b) Across the pinch c) Below the pinch

Figure 18. Different integration possibilities of heat engines in a network.

pinch

Ecool + (Q – W )

QEheat

Q – W W

Q

Eheat

HEN

above

pinch

HE HEN

belo

w

pinch

pinch

Q – W

Ecool – Q

Q

W

Eheat

HEN

above

pinch

HE HEN

below

pinch

pinch

Ecool

WEheat

Q – W

W

Q

Eheat – (Q – W)

HEN

above

pinch

HE

HEN

below

pinch

Q2

Q1

Q2

Q1

W W

High

Temperature

Source

High

Temperature

Source

Low Temperature

Source

Low Temperature

Source

Heat

Engine

Heat

Pump

21

If the heat engine is integrated so that it rejects heat to the network above the pinch

temperature (Fig. 18a), then heat will be transferred to the process heat sink. Therefore, network

hot utility demand will be reduced slightly and overall hot utility requirement is only increased

by engine shaft-work W. Such placement is appropriate because it implies 100% efficient heat

engine.

If the heat engine is placed so that it takes energy from the network below the pinch

temperature (Fig. 18c) then heat will be taken from overall process heat sources. It means that the

heat engine is running on free of fuel cost and overall cold utility requirement will be decreased

by engine shaft-work of W. Therefore, the heat engine is integrated appropriately.

If the heat engine is placed across the pinch temperature then both overall heat requirement

and overall cold requirement will be increased, as shown in Fig. 18b. Therefore, the heat engine

is integrated inappropriately.

To summarize, a heat engine placed either above the pinch or below the pinch are

appropriated, while a heat engine placed across the pinch is not appropriated.

6.2 Heat Pump (HP)

As pointed before a heat pump receives heat at a lower temperature and by using a

mechanical power rejects the heat at a higher temperature. The rejected heat is thus the sum of

input heat and the mechanical power. Key design parameters for heat pumps can be decided by

pinch analysis technology. Again, integration possibilities for heat pumps into a network process

can be done in three ways which are called as above the pinch, below the pinch and across the

pinch, see Fig. 19.

a) Above the pinch b) Across the pinch c) Below the pinch

Figure 19. Different integration possibilities of heat pumps in a network.

A heat pump can be placed so that it takes energy from the network above the pinch point and

reject heat to a higher temperature also above the pinch, see Fig. 19a. This integration possibility

decreases the overall heat demand by pump shift-work W in expense of an equal input power.

Therefore, such integration is inappropriate since the input power is more expensive than gained

heat.

In the second option, a heat pump can be integrated so that it takes energy from the network

below the pinch temperature and rejects heat to the temperature above the pinch, see Fig. 19b.

Such heat pump placement decreases the overall heating demand and overall cooling demand.

pinch

Ecool - Q

Q + W

W Q

Eheat – (Q + W)

HEN

above

pinch

HP HEN

belo

w

pinch

pinch

Q + W

Ecool + W

Q

W

Eheat

HEN

above

pinch

HP HEN

below

pinch

pinch

Ecool

Q + W

W

Q

Eheat – W

HEN

above

pinch

HP

HEN

below

pinch

22

The reduced overall heating and cooling demands is much bigger than the provided work to the

pump. Thus such integration possibility is appropriated.

The third option is that the heat pump is placed so that it takes energy from a point bellow the

pinch and rejects heat to a higher temperature which is also below the pinch, Fig. 19c. Such

integration increases the cooling demand with a value equal to the pump shaft-work W. This is

done in expense of an equal power provided for pump. Again, since the power is more expensive

than heat then such placement possibility is not appropriated.

To summarize, the only appropriate way to integrate a pump into a process is to place it

across the pinch. However, it is important to remind that the overall economic of a heat pump

depends on the heat saving when it is compared with the cost of input power, heat pump capital

cost and associated heat exchangers.

7. Process Modification by Plus – Minus Principle

In order to further decrease the overall energy requirement in a process, one needs to study

the composite curves (beside the process parameters such as operating pressure, operating

temperatures, reactor conversions, etc.). Since the composite curves strongly depend on the heat

and material balance of process, thus any changes in these parameters affect the composite

curves. Therefore, by applying the pinch rules mentioned above, one can easily identify the

changes in process parameter that have favorable impact on energy consumption.

The following rules are thus useful on studying the composite curves.

The hot utility target reduces by:

- Any increase (+) in hot streams duty above the pinch

- Any decrease (–) in cold streams duty above the pinch

The cold utility target reduces by:

- Any increase (+) in cold streams duty below the pinch

- Any decrease (–) in hot streams duty below the pinch

These simple rules are often called as “plus – minus principle” (+ / – principle) and are regarded

as guidelines for adjustment of a single heat duty. However, it is often possible to change process

temperature rather than its duty. The beneficial target for shifting the temperature can easily be

summarized as

Shift hot streams from below the pinch to above the pinch

Shift cold streams from above the pinch to below the pinch

These are in agreement with the general idea that it is beneficial to increase the hot streams

temperature (easier to extract energy) and reduce the cold streams temperature.

8. Pinch Technology and Future Possibilities

In late 1970s, the development of Pinch Technology started and it has found its place in

energy conversation applications. However, new developments for Pinch Analysis have been

done in other areas such as water use minimization, waste minimization, hydrogen management,

plastics manufacturing, etc. Below, some key research areas are pointed out.

Water Pinch: In view of rising fresh water costs and more stringent discharge regulations,

Pinch Analysis may help companies to systematically minimize freshwater consumption

and wastewater volumes. Water Pinch is a technique that can be used for analyzing water

networks and reducing water costs for processes.

23

Hydrogen Pinch: Hydrogen Pinch is the pinch technology approach which applies to

hydrogen management technique. Using Hydrogen Pinch, a designer may be able to set

targets for the minimum hydrogen production from the plant or hydrogen imports without

the need for any process design. Hydrogen Pinch may also help for effective use of

hydrogen purification units.

Top Level Analysis: Sometimes, it is very difficult to collect the required data in large

industrial areas. Which utilities are worth for saving can be determined by using a Top

Level Analysis with only efficiencies and constraints of the utility systems. Then data can

be gathered from those processes or units that use these utilities. Finally, a pinch analysis

can be performed on these equipment only.

Total Site Analysis: Refinery and petrochemical processes operate usually as part of large

factories or sites. These sites have several processes which are serviced by a centralized

utility system. Via the main steam supplement, there might be both consumption and

recovery of the steam for different processes. In such large sites, different departments

usually control the individual process and the central services. These departments usually

operate independently. To improve integration of total site infrastructure, a simultaneous

approach to consider individual process issues and entire site utility planning is

sometimes necessary. Pinch Technology can therefore be used to calculate energy targets

for the entire site.

Regional Energy Analysis: By studying the net energy demands of several companies

together (combined), the potential for sharing heat between these companies can be

identified. Such analyses may give insight into the amount of waste heat from these

industrial areas that might be available for export. Depending on the temperature level of

waste heat, it can also be used for power generation or district heating.

24

Assignments

Problem 1: In a process site the following process streams are given. Suppose Tmin = 0C.

Process stream

Nr / Type

Inlet Temp.

[C]

Outlet Temp.

[C]

Heat capacity rate

[kW/K]

1. cold 20 135 2.0

2. cold 80 140 6.0

4. hot 170 60 3.0

5. hot 150 30 1.5

a) Perform the temperature interval.

b) Perform the sequential and max table.

c) Find out the external heating and cooling demands.

d) What is the pinch temperature?

(Answer: Qheat = 95 kW

Qcool = 15 kW

Tpinch = 80C)

Problem 2: A chemical site is going to be designed based on the following process streams. It is

also desired to recover maximum amount of heat through a network of heat exchangers.

Process stream

Nr / Type

Inlet Temp.

[C]

Outlet Temp.

[C]

Heat capacity rate

[kW/K]

1. cold 100 430 16.0

2. cold 180 360 32.4

3. cold 200 400 29.7

4. cold 260 400 22.4

5. hot 440 150 28.0

6. hot 520 300 23.8

7. hot 390 150 33.6

Assume Tmin = 10C for the heat exchangers.

a) Find out the temperature interval.

b) Perform the cascade table.

c) What are the minimal external and cooling effects?

d) What is the pinch temperature?

e) Draw the composite curves.

f) Draw the grand composite curve.

g) Draw the grid diagram.

(Answer: Qheat = 216 kW

Qcool = 1448 kW

Tpinch = 200C)

25

Problem 3: Due to energy prices, a refinery is going to be modernized and at the same time

recover maximum energy in the site by using a complex network of heat exchangers.

Process stream

Nr / Type

Inlet Temp.

[C]

Outlet Temp.

[C]

Heat capacity rate

[kW/K]

1. cold 95 205 8.0

2. cold 40 220 8.0

3. cold 150 205 12.0

4. cold 65 140 20.0

5. hot 310 205 8.0

6. hot 245 95 13.0

7. hot 280 65 6.0

Answer to the following questions if Tmin for the heat exchangers is 10C. (Theoretical MER is

valid.)

a) Find out the temperature interval.

b) Draw the composite curves.

c) Perform the cascade table.

d) What are the minimal external and cooling effects?

e) What is the pinch temperature?

f) Draw the grid diagram for the heat exchangers.

g) Draw the grand composite curve.

(Answer: Qheat = 400 kW

Qcool = 0 kW

Tpinch = 40C)